Poleward moving auroral forms (PMAFs) observed at the Yellow River Station: A statistical study of its dependence on the solar wind conditions

摘要

Poleward moving auroral forms (PMAFs) are statistically investigated by using continuous, high-quality, optical observations for six winters obtained from three-wavelength all-sky imagers at the Chinese Yellow River Station (78.92°N, 11.93°E), Svalbard, in the Arctic. In particular, we examined the dependence of the PMAF occurrence on solar wind conditions in the magnetic local time (MLT) range of 0900-1500 MLT. Comparing to the previous studies, our larger PMAFs data base enables us to examine how the PMAFs' occurrence depends on each interplanetary magnetic field (IMF) component in different MLT sectors. It was found that 59%, 60%, and 57% of the PMAF events occurred under IMF conditions of negative Bz, positive By, and negative Bx, respectively. We also found that the PMAFs show a tendency to occur preferentially under large IMF |Bx| conditions. The MLT dependence of PMAFs presents a remarkable tendency of decreasing in the midday sector, showing a clear IMF By-related prenoon-postnoon asymmetry, which is consistent with the motion of reconnected flux tubes due to the release of magnetic field tension. For the first time, we found, however, that the asymmetry is more evident during conditions of Bz>0 than Bz<0, suggesting the importance of the effect of the lobe reconnection. Our statistical results suggest that the PMAFs are not caused by the solar wind pressure pulses, but by bursts of magnetic reconnection.